Neurophysiology. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nervous System Sensory Input – monitoring stimuli occurring

Neurophysiology

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Nervous System

Sensory Input – monitoring stimuli occurring

inside and outside the body

Integration – interpretation of sensory input

Motor Output – response to stimuli by activating

effector organs

Functions:


Organization of the Nervous System

PNS

Paired Spinal and Cranial nerves

Carries messages to and from the spinal cord

and brain – links parts of the body to the CNS

CNS

Brain and Spinal Cord (in dorsal body

cavity)

Integration and command center – interprets

sensory input and responds to input


PNS - Two Functional Divisions

Sensory (afferent) Division

Somatic afferent nerves – carry impulses from skin,

skeletal muscles, and joints to the CNS

Visceral afferent nerves – transmit impulses from

visceral organs to the CNS

Motor (efferent) Division

Transmits impulses from the CNS to effector

organs, muscles and glands, to effect (bring about)

a motor response


Motor Division: two subdivisions

Somatic Nervous System (voluntary)

Somatic motor nerve fibers (axons) that conduct

impulses from CNS to Skeletal muscles –

allows conscious control of skeletal muscles

Autonomic Nervous System (ANS) (involuntary)

Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands

Two functional divisions – sympathetic and parasympathetic


Levels of Organization in the Nervous System





Membrane Potentials: Signals

Two types of signals are produced by a change in

membrane potential:

graded potentials (short-distance)

action potentials (long-distance)


Graded Potentials

1-Short-lived, local changes in membrane potential

(either depolarizations or hyperpolarizations)

2-Cause currents that decreases in magnitude with

distance

3-Their magnitude varies directly with the strength of

the stimulus – the stronger the stimulus the more the

voltage changes and the farther the current goes

4-Sufficiently strong graded potentials can initiate

action potentials


Action Potentials (APs)

An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate.

The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV

APs do not decrease in strength with distance

The depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization

All-or-None phenomenon – action potentials either happen completely, or not at all


Propagation of an Action Potential

The action potential is self-propagating and

moves away from the stimulus (point of origin)


Stimulus Intensity

How can CNS determine if a stimulus intense or weak?

Strong stimuli can generate an action potential more

often than weaker stimuli and the CNS determines

stimulus intensity by the frequency of impulse

transmission

All action potentials are alike and are independent of stimulus intensity


Axon Conduction Velocities

Conduction velocities vary widely among neurons

Determined mainly by:

Axon Diameter – the larger the diameter, the faster

the impulse (less resistance)

Presence of a Myelin Sheath – myelination

increases impulse speed (Continuous vs. Saltatory

Conduction)


Saltatory Conduction

Current passes through a myelinated axon only at

the nodes of Ranvier

Voltage-gated Na+ channels are concentrated at

these nodes

Action potentials are triggered only at the nodes

and jump from one node to the next

Much faster than conduction along unmyelinated

axons


Saltatory Conduction

Current passes through a myelinated axon only at the nodes of Ranvier (Na+ channels concentrated at nodes)

Action potentials occur only at the nodes and jump from node to node

Erlanger and Gasser divided mammalian

nerve fibers into A, B, and C groups,

further subdividing the A group into α, β,

γ, and δ fibers.

NumbeNumberr

OriginOrigin Fiber TypeFiber Type

Ia Ia Muscle spindle, Muscle spindle, annulospinal ending. annulospinal ending.

A αA α

Ib Ib Golgi tendon organ. Golgi tendon organ. A αA α

IIIIMuscle spindle, flower-Muscle spindle, flower-spray ending; touch, spray ending; touch, pressure. pressure.

A βA β

III III Pain and cold receptors; Pain and cold receptors; some touch receptors.some touch receptors.

A δA δ

IV IV Pain, temperature, and Pain, temperature, and other receptors. other receptors.

Dorsal root CDorsal root C

Synapses


Synapse

A junction that mediates information transfer from

one neuron to another neuron

Presynaptic neuron – conducts impulses toward

the synapse (sender)

Postsynaptic neuron – transmits impulses away

from the synapse (receiver)


Types of Synapses

Axodendritic – synapse between the axon of one neuron and the dendrite of another

Axosomatic – synapse between the axon of one neuron and the soma of another

Other types:

Axoaxonic (axon to axon)

Dendrodendritic (dendrite to dendrite)

Dendrosomatic (dendrites to soma)


Synapses


Synapses can be…

Electrical

CHEMICAL!


Electrical Synapses

Less common than chemical synapses

Gap junctions allow neurons to be electrically

coupled as ions can flow directly from neuron to

neuron - provide a means to synchronize activity of

neurons


Electrical Synapse

Electrical synapses

gap junctions (connexins)

smooth and cardiac muscles, glial cells

Only a few examples of GJ have been found in the

central nervous system


Electrical Synapse


Chemical Synapse

o Functional connection between a neuron and another

neuron (or effector cell such as muscle, gland).One way conducton


Chemical Synapses

Specialized for the release and reception of chemical neurotransmitters

Typically composed of two parts:

Axon terminal of the presynaptic neuron containing membrane-bound synaptic vesicles

Receptor region on the dendrite(s) or soma of the postsynaptic neuron



Synaptic Cleft

Fluid-filled space separating the presynaptic and

postsynaptic neurons, prevents nerve impulses from

directly passing from one neuron to the next

Transmission across the synaptic cleft:

Is a chemical event (as opposed to an electrical

one)

Ensures unidirectional communication between

neurons




Synapse

AP comes down the axon.

◦ At the synapses, VG Ca++

channels let in calcium.

◦ This triggers the release

(exocytosis!) of the contents

of vesicles in the axonal bouton.

◦ The contents are:

NEUROTRANSMITTERS.


Synapse

Neurotransmitters (NT) cross the narrow synaptic

space and bind to receptors on the dendrite (or other

cell).

This causes a response in the postsynaptic cell.

The whole cycle starts again in this second cell!


Postsynaptic Potentials

EPSP (excitatory postsynaptic potential):Depolarization.Brings cell closer to threshold for an AP.Often Na+ channels.

IPSP (inhibitory postsynaptic potential):Hyperpolarization.Takes cell further away from threshold for an AP.Often Cl- and K+ channels.


Excitatory Postsynaptic Potentials

EPSPs are local graded depolarization events that can initiate an action potential in an axon

Postsynaptic membranes do not generate action potentials. The currents created by EPSPs decline with distance, but can spread to the axon hillock and depolarize the axon to threshold leading to an action potential


Inhibitory Postsynaptic Potentials

Neurotransmitter binding to a receptor at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an action potential

Postsynaptic membrane is hyperpolarized due to increased permeability to K+ and/or Cl- ions. Leaves the charge on the inner membrane face more negative and the neuron becomes less likely to “fire”.



Summation

IPSPs also summate and can summate with EPSPs.

Temporal Summation – presynaptic neurons transmit impulses in quick succession

Spatial Summation – postsynaptic neuron is stimulated by a large number of terminals at the same time

A single EPSP cannot induce an action potential EPSPs must summate (add together) to induce an AP


Summation


EPSP



So…

AP flies down axon of first neuron.

NT are released at synapse.

Receptors bind NT and produce an EPSP or

IPSP in postsynaptic neuron.

The sum of the inputs -> AP in this second

neuron.


Neurotransmitters

Chemicals used for neuron communication with

the body and the brain

More than 50 different neurotransmitters have

been identified

Classified chemically and functionally


Neurotransmitters

Small molecules,

Rapidly acting

Cause most acute responses of the nervous system such

as:

o Transmission of sensory signals to the brain

o Transmission of motor signals to the muscles


Small molecules Rapidly acting

Synthesized in presynaptic terminals

Absorbed by means active transport to the vesicle


Neurotransmitters – Chemical classification

•Acetylcholine (ACh)

•Biogenic amines

•Amino acids

•Peptides

•Novel messengers: ATP and dissolved gases

NO and CO


Neurotransmitters: Acetylcholine

Released at the neuromuscular junction

Enclosed in synaptic vesicles

Degraded by the acetylcholinesterase (AChE)

Released by:

All neurons that stimulate skeletal muscle

Some neurons in the autonomic nervous system


Neurotransmitters: Biogenic Amines

Include:

Catecholamines – dopamine, norepinephrine, and

epinephrine

Indolamines – serotonin and histamine

Broadly distributed in the brain

Play roles in emotional behaviors and our biological

clock


Neurotransmitter Receptor Mechanisms

Direct: neurotransmitters that open ion channels

◦ Promote rapid responses

◦ Examples: ACh and amino acids

Indirect: neurotransmitters that act through second

messengers

◦ Promote long-lasting effects

◦ Examples: biogenic amines, peptides, and dissolved

gases



GABAA Receptor


Termination of Neurotransmitter Effects

Neurotransmitter bound to a postsynaptic neuron

produces a continuous postsynaptic effect and also

blocks reception of additional “messages”

Terminating Mechanisms:

1- Degradation by enzymes

2- Uptake by astrocytes or the presynaptic terminals

3- Diffusion away from the synaptic cleft


Neuropeptides

Large molecules, slowly acting

Cause more prolonged actions such as:

1. Long term changes in number of neuronal receptors

2. Long term opening / closure or of certain ion channels

3. Long term changes in numbers of synapses or sizes of

synapses


Neuropeptides:

Are generally thousand or more times as

potent as small molecules


Neuropeptides

Are synthesized by ribosome in cell body

The vesicles are transported to the terminal

Much smaller quantities released than

small molecules


Neuropeptide Transmission


Synaptic Delay

Neurotransmitter must be released, diffuse across

the synapse, and bind to receptors (0.3-5.0 ms)

Synaptic delay is the rate-limiting step of neural

transmission


Fatigue of synaptic transmission

Exhaustion or partially exhaustion of neurotransmitter

stores

Progressive inactivation of postsynaptic receptors

Slow development of abnormal concentrations of ions

inside the postsynaptic neuron


Effect of acidosis and alkalosis on synaptic transmission

Alkalosis increases neuronal excitability

-Overbreathing can precipitate an epileptic attack

Acidosis depresses the neuronal activity

-in very sever diabetic acidosis, coma always develops


Effect hypoxia on synaptic transmission

Neuronal excitability is highly dependent on adequate

supply of oxygen

Cessation of oxygen for only a few seconds can cause

inexcitability of some neurons

When brain blood flow interrupted the person becomes

unconscious


Effect drugs on synaptic transmission

Caffeine (found in the coffee), theophylline( tea) and

theobromine(cocoa) increase neuronal excitability

-By reducing threshold for excitation of neurons

• Anesthetics increase threshold

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Neurophysiology. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nervous System Sensory Input – monitoring stimuli occurring